1. Field of the Invention
The present invention relates to a permanent magnet for magnetic bias used for a magnetic core (hereafter, may be briefly referred to as xe2x80x9ccorexe2x80x9d) of an inductor component, for example, choke coils and transformers. In particular, the present invention relates to a magnetic core, that is, a low-profile magnetic core capable of reducing the thickness of the inductor component.
2. Description of the Related Art
Regarding conventional choke coils and transformers used for, for example, switching power supplies, usually, the alternating current is applied by superimposing on the direct current. Therefore, the magnetic cores used for these choke coils and transformers have been required to have an excellent magnetic permeability characteristic, that is, magnetic saturation with this direct current superimposition does not occur (this characteristic is referred to as xe2x80x9cdirect current superimposition characteristicxe2x80x9d).
As high-frequency magnetic cores, ferrite magnetic cores and dust cores have been used. However, the ferrite magnetic core has a high initial permeability and a small saturation magnetic flux density, and the dust core has a low initial permeability and a high saturation magnetic flux density. These characteristics are derived from material properties. Therefore, in many cases, the dust cores are used in a toroidal shape. On the other hand, regarding the ferrite magnetic cores, the magnetic saturation with direct current superimposition has been avoided, for example, by forming a magnetic gap in a central leg of an E type core.
However, since miniaturization of electronic components is required accompanying recent request for miniaturization of electronic equipment, magnetic gaps of the magnetic cores must become small, and requirements for magnetic cores having a high magnetic permeability for the direct current superimposition have become intensified.
In general, in order to meet this requirement, magnetic cores having a high saturation magnetization must be chosen, that is, the magnetic cores not causing magnetic saturation in high magnetic fields must be chosen. However, since the saturation magnetization is inevitably determined from a composition of a material, the saturation magnetization cannot be increased infinitely.
A conventionally suggested method for overcoming the aforementioned problem was to cancel the direct current magnetic field due to the direct current superimposition by incorporating a permanent magnet in a magnetic gap formed in a magnetic path of a magnetic core, that is, to apply the magnetic bias to the magnetic core.
This magnetic bias method using the permanent magnet was superior method for improving the direct current superimposition characteristic. However, since when a metal-sintered magnet was used, an increase of core loss of the magnetic core was remarkable, and when a ferrite magnet was used, the superimposition characteristic did not be stabilized, this method could not be put in practical use.
As a method for overcoming the aforementioned problems, for example, Japanese Unexamined Patent Application Publication No. 50-133453 discloses that a rare-earth magnet powder having a high coercive force and a binder were mixed and compression molded or compacted to produce a bonded magnet, the resulting bonded magnet was used as a permanent magnet for magnetic bias and, therefore, the direct current superimposition characteristic and an increase in the core temperature were improved.
However, in recent years, requirements for the improvement of power conversion efficiency of the power supply have become even more intensified, and regarding the magnetic cores for choke coils and transformers, superiority or inferiority cannot be determined based on only the measurement of the core temperature. Therefore, evaluation of measurement results using a core loss measurement apparatus is indispensable. As a matter of fact, the inventors of the present invention conducted the research with the result that even when the resistivity was a value indicated in Japanese Unexamined Patent Application Publication No. 50-133453, degradation of the core loss characteristic occurred.
Furthermore, since miniaturization of inductor components has been even more required accompanying recent miniaturization of electronic components, requirements for low-profile magnet for magnet bias have also become intensified.
In recent years, surface-mounting type coils have been required. The coil is subjected to a reflow soldering treatment in order to surface-mount. Therefore, the magnetic core of the coil is required to have characteristics not being degraded under this condition. In addition, a rare-earth magnet having oxidation resistance is indispensable.
Accordingly, it is an object of the present invention to provide a magnetic core using a magnet for magnetic bias especially having a capability to miniaturize the magnetic core. The magnetic core has at least one gap in a magnetic path of a miniaturized inductor component, and has a permanent magnet as a magnet for magnetic bias in the neighborhood of the gap in order to apply magnetic bias to the magnetic core from both ends of the gap.
It is another object of the present invention to provide a magnetic core having superior direct current superimposition characteristic and core loss characteristic, with ease at low cost. Furthermore, the magnetic core has oxidation resistance and, therefore, the characteristics are not affected even under the reflow conditions.
It is still another object of the present invention to provide, in consideration of the above description, a magnetic core having superior direct current superimposition characteristic and core loss characteristic with ease at low cost regarding the magnetic core having at least one gap in a magnetic path, and having a permanent magnet as a magnet for magnetic bias in the neighborhood of the gap in order to apply magnetic bias to the magnetic core from both ends of the gap.
It is yet another object of the present invention to provide a miniaturized inductor component.
According to an aspect of the present invention, there is provided a magnetic core which includes at least one gap in a magnetic path and a permanent magnet inserted into the gap, has an alternating current magnetic permeability at 20 kHz of 45 or more in a magnetic field of 120 Oe under application of direct current, and has a core loss characteristic of 100 kW/m3 or less under the conditions of 20 kHz and the maximum magnetic flux density of 0.1 T.
According to another aspect of the present invention, there is provided an inductor component which includes the aforementioned magnetic core, and at least one turn of coil is applied to the magnet core.